This application claims priority to Swedish Application No. 0003681-4, filed Oct. 12, 2000.
1. Technical Field
The present invention relates to safety devices. More specifically, the invention relates to a device for pretensioning a safety belt web joined to a reel mechanism in a vehicle.
2. Background Information
Belt pretensioners are typically used to take up slack in motor vehicle safety belts, e.g., in a collision, between the web windings on the spool of the reel mechanism. At the same time, pretensioners tension up the web against the passenger""s body in order to prevent, as much as possible, sliding under the belt or jerking back and forth, resulting in whiplash. It is known that the heavier the person, the higher the pretensioning force required for eliminating slack in the belt. Further, pretensioning which is too forceful can injure a small, light person.
Various devices and methods for adapting the pretensioning force to the weight of the passenger are known. For example, DE-OS 38 17 942 teaches two different solutions, both based on controlling pressure in a cylinder chamber at expansion by the drive gas formed at detonation, whereby the pressure for a lighter person is lower than that for a heavier person. In both solutions, a mechanical device found in the seat is utilized. This device reacts to pressing down the seat cushion. In one case, the volume of the cylinder chamber is controlled by the mechanical device displacing a movable wall in the chamber. If the person is heavy, the wall is set so that the volume will be small. If the person is light, the volume is increased so that the force on the person will be less. In the latter case, a valve slide is controlled relative to a number of evacuation openings from the cylinder chamber so that gas in the chamber is evacuated earlier the lighter the person is. In the first case, the weight adaptation of the pretensioning is infinitely variable, while in the second case it is effected in six steps.
DE 296 12 781 U1 teaches a device for weight adaptation of the belt pretensioning in only two steps. Here, a piston-cylinder device is used with two propulsion charges, both of which are set off if the person on the seat is heavy. If the person is light, only one of the propulsion charges is set off.
In the above known solutions, the piston-cylinder devices are pulling, i.e., the starting position is with the piston rod out of the cylinder. Consequently, the expansion volume for the propulsion charges will be situated on the piston rod side of the piston. Therefore, during expansion the piston rod is drawn into the cylinder.
With the above solutions, complete sealing between the piston rod and the opening in the cylinder wall is difficult to achieve. Therefore, leakage of propellant gas is practically unavoidable. Accordingly, there is a need for a pretensioner that avoids the problem of propellant gas leakage.
The pretensioner of the present invention includes a cylinder and a piston displaceable in the cylinder and having a piston rod. The piston rod is joined to one side of the piston and extends through an opening in one end wall of the cylinder. The pretensioner further includes a motion-transmitting element acting between the piston rod and a reel mechanism. Upon displacement of the piston in one direction, the element achieves rotation of the reel mechanism in the winding-up direction of the belt web. The pretensioner further includes both a pyrotechnic charge that, when detonated, causes a pressure increase in a cylinder chamber between the piston and a cylinder end wall for displacement of the piston in one direction, and a controller for controlling the pressure in the cylinder chamber depending on the weight of the person held in by the belt.
One purpose of the present invention is to achieve a device of the type described above that makes possible secure precision adaptation of the pretensioning to the weight of the passenger. An additional purpose of the invention is to achieve a device which eliminates the risk of propellant gas leaking out between the piston rod and the cylinder opening through which the piston rod extends.
This is achieved according to the invention by virtue of the fact that the cylinder chamber is delimited between the side of the piston opposite to the piston rod and the second end wall of the cylinder. Further, the motion-transmitting element is arranged to rotate the reel mechanism in the winding-up direction of the belt web when the piston is displaced in the projection direction of the piston rod. The cylinder chamber has an outlet that communicates with a spill valve, which can be set between various degrees of opening and a completely closed position.
By rotating the reel mechanism in the winding-up direction of the belt, thereby using a pushing instead of a pulling movement of the piston rod, the expansion chamber is placed on that side of the piston which does not have any through-hole for the piston rod. This eliminates one source of leakage for the propellant gas. By combining this arrangement with a spill valve that can be set at a completely closed position, the piston-cylinder device can be blocked in the pretensioned position.
In a preferred further development of the device according to the invention, this possibility is utilized in combination with a positional sensor coupled to the control unit, or xe2x80x9croll-over sensorxe2x80x9d. This sensor senses if the vehicle is upside-down, thereby providing a signal to the control unit to close the spill valve, thus blocking the device in the pretensioned position so that the passenger is held securely against the seat and will not risk hitting his head against the vehicle ceiling.
In a further development of the device according to the invention, the motion-transmitting element is joined to and partially wound up on a drum. This drum is solidly joined to a ring gear of a planetary gear set, so disposed and coupled to the reel mechanism that the gear ratio between the ring gear and the reel mechanism is 1:1 when the ring gear rotates in the winding-up direction of the belt web, and is geared up upon rotation in the opposite direction so that winding up of a certain length of the motion-transmitting element on the drum of the ring gear corresponds to a several times longer feeding-out of the web from the reel mechanism.
In previously known designs, the force-limiting function is achieved by plastic deformation of a torsion element in the reel mechanism. This function occurs subsequent to pretensioning upon reeling out of the belt due to the load caused by vehicle retardation from the passenger""s body against the belt. In practical embodiments, the feed-out length of the belt is, for design reasons, limited to approximately 300 mm. By utilizing a planetary gear set between the motion-transmitting element and the reel mechanism, the feed-out length can be increased by at least fifty percent without any problem.